The invention applies more particularly to the double-column distillation of large air throughputs.
The dimensions of double distillation columns, in particular their maximum diameters corresponding to their low-pressure columns, increase with the air throughputs which they are capable of distilling.
Above a certain air throughput, typically 600,000 m.sup.3 [stp]/h, the dimensions of a double column generally no longer allow it to be transported.
Two solutions have so far been adopted for constructing an air distillation plant capable of treating large air throughputs on an industrial site distant from the works of a company constructing the column.
The first solution consists in creating a column construction workshop on the industrial site in order to construct one double column with dimensions sufficient for treating the air throughputs in question.
Such a solution involves setting up elaborate temporary logistics, and is found to be particularly expensive.
The second solution consists in producing, in the workshop, a plurality of double distillation columns whose dimensions allow them to be transported, then transporting them to the industrial site where they are installed in parallel as a corresponding number of individual distillation units allowing the air throughputs in question to be treated separately.
Each double column installed on the site is thus connected to its own air purification device, its own heat-exchange line, and is surrounded by its own thermal insulation wall, thus forming as many cold boxes as there are double columns. Such a solution is also expensive.
GB-A-1216192 describes a system for distilling air to produce separate streams of oxygen at two different purities using a medium pressure column to produce reflux streams for two low pressure columns. The medium pressure column is thermally linked with one of the low pressure columns via a reboiler-condenser; this column is additionally fed by expanded air.